The present invention relates to a process for producing an aromatic polycarboxylic acid by oxidizing a corresponding polyalkyl-substituted aromatic aldehyde or a polyalkyl-substituted carboxylic acid in water as a solvent in the presence of a bromine ion-containing catalyst.
Among aromatic polycarboxylic acids, terephthalic acid is used as raw materials for synthetic fibers and synthetic resin, trimellitic acid is widely used as raw materials for alkyd resin, high grade plasticizer and polyester, and pyromellitic acid is used as raw materials for special plasticizer, polyamide and polyimide.
The so far well known processes for producing aromatic polycarboxylic acids include a process for producing terephthalic acid by oxidizing p-xylene with air in an acetic acid as a solvent in the presence of a cobalt-manganese-bromine catalyst, a process for producing trimellitic acid by oxidizing pseudocumene in the same manner as in the said process for producing terephthalic acid, or by oxidizing pseudocumene with nitric acid, and a process for producing pyromellitic acid by oxidizing a polyalkyl-substituted benzene such as durene, trimethylisopropylbenzene, etc. in a gaseous phase or with nitric acid.
As a result of extensive studies of a process for economically producing aromatic polycarboxylic acids such as trimellitic acid, pyromellitic acid, etc., the present inventors found that aromatic polycarboxylic acids could be obtained easily in high yields in one step by oxidizing the corresponding polyalkyl-substituted aromatic aldehydes or polyalkyl-substituted aromatic carboxylic acids with molecular oxygen in water as a solvent in the presence of bromine ions and metal ions of manganese, cerium, etc., as already disclosed in Japanese Patent Application Kokai (Laid-Open) No. 26839/81, but the prior process is an improved advantageous oxidation process free from the disadvantages of the conventional processes, but still has such a disadvantage as a high corrosiveness due to a reaction condition involving bromine ions and molecular oxygen at a high temperature.
On the other hand, Japanese Patent Application Kokai (Laid-Open) No. 125631/79 discloses a process for producing terephthalic acid by oxidizing p-tolualdehyde in water as a solvent in the presence of bromine ions in a reactor using zirconium as a material of construction, which still has a risk of occurrence of corrosion at an elevated reaction temperature, or under an elevated oxygen partial pressure or at an elevated concentration of Br.sup.- as the catalyst, particularly at an elevated HBr concentration, where it has been found that the corrosion is not a general corrosion occurring on the entire surface of liquid-contact parts of the material of construction, but a local corrosion such as pitting corrosion, intercrystalline corrosion, etc.
Generally, the pitting corrosion is characteristic of very small cross-sectional area of corrosion and of corrosion advancing deeply in a material, and thus has such a risk that a pitting hole penetrates a material as the corrosion advances, though the cross-sectional area of corrosion is very small. Particularly in the case of using a corrosion resistant material in a pressure vessel as in the present invention, the ordinary corrosion resistant material having a thickness of several millimeters on average is used as a lining material or cladding material to a low grade substratum material such as carbon steel, and thus occurrence of pitting corrosion must be completely prevented, if a possible accident due to the occurrence and the successive advance of a local corrosion is taken into account.
As a result of further studies of a material for a reactor for producing an aromatic polycarboxylic acid without any risk of such local corrosion and of corrosion tests of zirconium under expected oxidation reaction conditions, the present inventors have found that the occurrence of corrosion can be considerably reduced or prevented by using zirconium whose surface is coated with an oxide layer as a material of construction for the reactor, and have established the present invention.